The present invention relates to a tool which provides a complete directional survey of an oil or gas well borehole for determining the displacement of the borehole path in all three directions relative to the well head at the surface.
Most instruments or tools which are used to perform directional surveys include devices which measure the attitude, or angular orientation, of the tool as it is lowered on a cable into the borehole. Typically, accelerometers or tilt sensors are used to measure tool inclination and gyroscopic or magnetic compasses are employed to locate North. The displacement of the tool is calculated by projecting each incremental increase in cable length in the direction determined by the tool attitude. Displacement measurement errors are usually on the order of ten feet for each one thousand feet of borehole length and result from errors in the gravity and north sensing devices, and misalignment between the tool and the borehole. In addition, displacement measurement errors result due to inaccuracies in the cable length measurement.
Recently, efforts have been made to adapt aircraft inertial navigation systems for use as borehole directional surveying instruments. In an inertial navigation system the outputs of the accelerometers and gyroscopes are manipulated in a computer to provide velocity and displacement. No measurements of cable length are required.
Broadly speaking, two types of navigation system mechanization exists, namely, gimballed and strapped down. In a gimballed system, the sensor block containing the gyroscopes and accelerometers is supported by a set of gimbals which isolate the sensor block from rotation relative to the outer casing. In normal operation, the sensor block remains virtually rotationally motionless and maintains approximately its initial or referenced alignment. The outputs of gyroscopes on the sensor block are used in null-seeking servo loops which drive the gimbals so that no rotation of the sensor block occurs. An advantage of a gimballed system is that the gyroscopes are not required to measure large angular rates. The gyroscopes operate in a benign, nominally zero, angular rate environment. An important disadvantage of gimballed systems is high mechanical complexity.
A strapped-down system does not include gimbals. Any angular rate of rotation of the instrument casing within the borehole is transmitted directly to the sensor block. The gyroscopes must measure a wide range of angular rates from zero up to perhaps several hundred degrees per second. Strapped-down systems are mechanically simpler and usually smaller than gimballed systems.
Hithertofore, a number of borehole sensor devices have been developed which normally include a gyroscope and accelerometers to measure the attitude and a cable measurement device to determine the depth of the instrument within the borehole. For example, two patents issued to Van Steenwyk, U.S. Pat. No. 3,753,296 and U.S. Pat. No. 4,199,869, together with the Van Steenwyk et al apparatus which employs an attitude gyroscope mechanism. Each of these patents includes a suitable meter 17 which records the length of the cable extending downwardly in the well for logging purposes. In addition, each patent discloses a gyroscope 25 which is mounted on slip ring structures 25a and 26a being provided with terminals for transmitting signal outputs of the gyroscope and an accelerometer 26.
Barriac, U.S. Pat. No. 4,244,116, discloses a device for measuring the azimuth and the slope of a drilling hole. This device incorporates a gyroscope 5 and an accelerometer 12 which include two principle axes of sensitivity which are parallel with respect to each other. This device is designed only for measuring the attitude of a borehole.
Barriac, U.S. Pat. No. 4,238,889, discloses a device for scanning the azimuth and slope of a borehole. The device includes a gyroscope and an accelerometer which are suspended from a cable. The gyroscope and accelerometer are only for measuring the attitude of the mechanism within the borehole and a measuring means 6 is provided for measuring the length of the cable as the device is lowered into the borehole.
Armistead, U.S. Pat. No. 3,691,363, discloses a method and apparatus for directional logging of a borehole. A constant speed electric motor 11, for example, a synchronous motor, is mounted in an upper instrument casing 13 to rotate a first coil 10. The first coil 10 is positioned within a Helmholtz coil 40. The Helmholtz coil 40 is gimbally mounted to the upper instrument case 13 so that it maintains its central axis in a vertical orientation whereby the rotating inclined coil 10 is subjected to magnetic flux lines of a substantially vertical direction. A length measuring and pulse generating unit 119 is provided which is mechanically coupled with the sheave 114 and includes means for measuring each increment of cable length passing over the sheave 114 as the logging instrument is raised or lowered. The length measuring and pulse generating unit 119 issues a trigger pulse for each length increment measured.
The Grosso et al Patent, U.S. Pat. No. 3,982,431, and the Asmundsson et al Patent, U.S. Pat. No. 4,021,774, disclose borehole sensor devices. Each borehole sensor includes a three axis gimbal device for determining a vertical plane using the force of gravity as a reference, a horizontal plane using the force of gravity as a reference and a north direction using the magnetic field as a third axis reference. Both of these devices utilize mud which flows through the orifice 50 which creates pressure pulses in the mud stream which are transmitted to and sensed at the surface to provide indications of various conditions sensed by the sensor unit 44. The mechanisms disclosed in both of these patents are actually operative for a short period of time every thirty feet or so of depth of the borehole. The device disclosed in both of these patents is utilized to measure the attitude of the mechanism within the borehole.
Poquette, U.S. Pat. No. 4,245,498, discloses a well surveying instrument which provides incremental angular information about two axes normal to the axial center line of a well pipe to be surveyed. Tilt information is provided by a rate gyro positioned in azimuth by a null-seeking azimuth gyro on an azimuth gimbal. The depth of the device within the borehole is obtained by measuring the length of the cable paid out as the device is lowered into the borehole.
Starr, U.S. Pat. No. 4,302,886, discloses a gyroscopic directional surveying instrument. The subject matter set forth in the Starr Patent is primarily directed to protecting the instrument disposed within the outer casing of the device. More particularly, Starr provides a surveying instrument which can withstand pressure of 24,000 pounds per square inch. In addition, the device is designed to withstand temperature in the range of 450.degree. F. The instrument disclosed by Starr includes a vacuum flask and a pressure vessel in which the instruments are disposed.